This invention relates in general to brake systems for motor vehicles, and in particular to fluid pressure operated brake boosters for vehicle braking systems.
Conventional vehicle braking systems are constructed such that the braking force developed by the system is generally proportional to the force applied by the vehicle operator on a vehicle brake pedal. The pedal is linked to a piston in a master cylinder which moves to pressurize the fluid of the brake system, and thus actuate the individual wheel brakes. Generally, in order to provide a sufficiently high pressure to operate the wheel brakes without requiring an excessive effort by the operator, most vehicles include a "boosted" power brake system wherein the force applied to the brake pedal by the operator is amplified or boosted before being applied to the master cylinder. In automobiles and light trucks, this is typically accomplished by incorporating either a vacuum or hydraulically operated boost piston assembly between the brake pedal and the master cylinder.
Vacuum operated boost piston assemblies utilize the pressure differences between the atmosphere and the vehicle's engine vacuum to develop a force across a boost piston or diaphragm. The force developed by this differential pressure across the boost piston may be many times that which could be developed by the operator by pressing against the pedal, and is applied to the piston of the master cylinder to actuate the brakes.
Hydraulic boost piton assemblies utilize a source of pressurized fluid developed by a hydraulic system, for example, using the vehicle's power steering pump, to apply a force to a boost piston or a plurality of boost pistons. The force applied by the hydraulic system to the boost piston, like the vacuum boost system, may be many times that which could be applied by the operator of the vehicle when operating the brake pedal through the same stroke length used to actuate the hydraulic boost piston assembly. Thus, the increased force applied to the piston of the master cylinder generates higher braking system operating pressures for a given pedal force input.
For some hydraulic boosters, undesirable boost piston travel length can occur when the fluid pressure source fails, such as in an engine stall in a brake circuit utilizing a power steering pump. Upon failure, the travel length of a boost piston and accordingly the brake pedal, may need to be advanced, during which there is generally not an increase in brake pressure generated by the master cylinder. This extraneous travel length is generally undesirable to the operator of the vehicle. Undesirable boost piston travel length can also occur after the demand for pressurized fluid flowing into the hydraulic booster rises above the maximum output of the source of pressurized fluid, referred to as booster runout. Thus, it would be desirable to provide a brake booster having a piston travel limit feature, to help prevent undesirable boost piston travel during booster runout and hydraulic system failures.